How important the review source is depends on how many people who read those reviews would be likely to buy the game anyways ...
A magazine like O (the official Oprah magazine) has so little overlap with the target demographic of Killzone 2 that its review score would be unimportant to the game. In contrast, Maxim's readership probably has a fairly large overlap with the target demographic of Killzone 2 and will hit many gamers who do not read reviews from sites like IGN (and don't know about metacritic or gamerankings).